Proteins with a CaaX motif regulate a number of pathways important in oncogenesis. These proteins undergo a series of post-translational modifications that are important for their localization, stability and function. The modifications are initiated by the addition of an isoprenoid moiety (farnesyl or geranylgeranyl) to the cysteine of the CaaX motif by protein farnesyltransferase (FTase) or protein geranylgeranyltransferase-1 (GGTase-1) respectively. This is followed by the endoproteolytic release of the terminal tripeptide (AAX) by RAS converting enzyme (RCE1) and carboxylmethylation of the C-terminal prenylcysteine by isoprenylcysteine carboxyl methyltransferase (Icmt).
The most widely studied example of CaaX proteins is the RAS family of regulatory proteins. RAS is a very important molecular switch for a variety of signaling pathways that control diverse processes like cytoskeletal integrity, proliferation, cell adhesion, apoptosis and cell migration. Activating mutations in RAS genes are implicated in the pathogenesis of a large number of solid tumors and hematologic malignancies. Many cancers contain alterations upstream of RAS in signaling cascades and the resultant hyperactivation of RAS is thought to contribute to tumorigenesis.
The possibility of blocking RAS-induced oncogenic transformation by inhibiting the enzymes involved in the post-translational processing of the CaaX motif has been explored for its therapeutic potential. The protein prenyltransferases in particular FTase have been targets of major drug discovery programs. FTase inhibitors showed significant activity in mouse models but clinical trials in cancer patients had been disappointing, possibly due to the geranylgeranylation of substrates by GGTase1 when FTase is inhibited. Hence, attention has been shifted to the post-prenylation enzymes RCE1 and Icmt as potential therapeutic targets. In particular, there is keen interest in developing Icmt inhibitors in view of studies that showed that genetic and pharmacological intervention with Icmt activity led to significant impairment of oncogenesis in several tumor cell models.
To date, three classes of Icmt inhibitors have been investigated. The first class comprises of S-adenosylhomocysteine (AdoHcy) and compounds that increase intracellular AdoHcy. AdoHcy is formed when a methyltransferase catalyzes the transfer of the methyl group from S-adenosylmethonine (AdoMet) to the substrate. AdoHcy binds to and competitively inhibits methyltransferase activity. However, AdoHcy is not a selective inhibitor of Icmt and affects the activity of other cellular methyltransferases. The second class of Icmt inhibitors is structural analogues of the substrate prenylcysteine. Examples are N-acetyl-S-farnesyl-L-cysteine (AFC) and N-acetyl-S-geranylgeranyl-L-cysteine (AGGC). These compounds are competitive inhibitors of Icmt but as structural mimics of the carboxy-terminal prenylcysteine of processed CaaX proteins, they would impact a large number of processes controlled by CaaX proteins.
Cysmethynil (2-[5-(3-methylphenyl)-1-octyl-1H-indolo-3-yl]acetamide) is a competitive inhibitor of the isopenylated cysteine substrate and a non-competitive inhibitor of the methyl donor AdoMet. Cysmethynil caused the mislocation of RAS and impaired epidermal growth factor signaling in cancer cells. It blocked anchorage-independent growth in a colon cancer cell line which was reversed by overexpression of Icmt. A recent report showed that induction of autophagy by cysmethynil is a major contributor to the cell death that accompanies pharmacological inhibition of Icmt.
Some natural products from marine sponges (spermatinamine, aplysamine 6) and plants (prenylated β hydroxychalcones, a flavanone S-glabrol) have been identified as Icmt inhibitors but they were either weakly potent inhibitors or lack drug-like features. Cysmethynil remains the most promising compound to date. However, cysmethynil has no dissociable functionalities and its high lipophilicity and poor aqueous solubility may potentially restrict its clinical application. Although modifications made at the N-substituent of the indole ring and the phenyl substituents of cysmethynil have been studied, the clinical relevance of these analogs is unknown.
Therefore, there remains a need to identify effective inhibitors of Icmt that can have potential therapeutic effects for treating cancer and diseases or disorders associated with Icmt activity.
It is a further object of the invention to provide a compound that can be a potent inhibitor of Icmt.
Another object of the invention is to provide a pharmaceutical composition that can have an anti-proliferative effect on cancer cells.